L-Ornithine has been shown to inhibit the development of cytotoxic T lymphocytes (CTL) in mixed lymphocyte cultures (MLC). The inhibitory effect is selective since cell proliferation in MLC, the mitogen-stimulated production of IL-2, IL-3 and interferon, the IL-2 dependent proliferation of T-cell clones or mitogen-activated thymocytes, and the activation of suppressor T cells are unaffected. Inhibition of CTL Is not due to toxic effects. MLC washed free of ornithine after three days produce control levels of CTL activity within 48 hours. Lymphokines are unable to reverse the inhibitory effect, and ornithine added only during the effector phase has no effect on cytolysis of target cells. In lymphocyte populations depleted of accessory cells (AC) so that CTL were dependent on exogenous lymphokines, induction of CTL remained sensitive to ornithine-mediated inhibition. Thus, in the ornithine-inhibited MLC T helper cell and AC functions appear normal; the effects of ornithine are mediated directly upon the CTL precursors (CTLp), which undergo clonal activation, but whose maturation is arrested at a precytolytic stage (CTLp*). Arginine and putrescine also suppress CTL generation, consistent with a hypothesis that the inhibitory effect of ornithine is mediated by polyamines which are known to influence the growth and development of a variety of cells. The selectivity of the inhibition will facilitate studies analyzing the process of CTL differentiation. The objectives of this investigation focus on characterizing the state of activation of the ornithine-inhibited CTLp*, and testing the hypothesis that polyamines play a role in mediating inhibition. Toward these objectives we will: 1) Quantitate the clonal expansion of CTLp in ornithine-inhibited MLC. 2) Determine whether differentiation of CTLp* into killer cells requires DNA synthesis, protein synthesis, and helper factors. 3) Analyze CTLp* for antigen binding, Lyt 1 and T145 cell surface markers. 4) Test products and antagonists of polyamine biosynthesis for effects on CTL activation. 5) Test products and antagonists of polyamine biosynthesis on reactivation of CTL memory cells, cytolytic activity of cloned CTL lines, and activation of CTL-hybridomas with inducible cytolytic activity. Ornithine-mediated arrest of CTL maturation provides an important model to study biochemical regulatory pathways in CTL development. Elucidation of the mechanism of suppression could lead to therapeutic applications by selective immunoregulation of CTL activation in transplantation and cancer.